siox: add gpio bus driver

This bus driver uses GPIOs to control the four SIOX bus lines.

Acked-by: Gavin Schenk <g.schenk@eckelmann.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/siox/eckelmann,siox-gpio.txt

@@ -0,0 +1,19 @@
+Eckelmann SIOX GPIO bus
+
+Required properties:
+- compatible : "eckelmann,siox-gpio"
+- din-gpios, dout-gpios, dclk-gpios, dld-gpios: references gpios for the
+    corresponding bus signals.
+
+Examples:
+
+        siox {
+                compatible = "eckelmann,siox-gpio";
+                pinctrl-names = "default";
+                pinctrl-0 = <&pinctrl_siox>;
+
+                din-gpios = <&gpio6 11 0>;
+                dout-gpios = <&gpio6 8 0>;
+                dclk-gpios = <&gpio6 9 0>;
+                dld-gpios = <&gpio6 10 0>;
+        };